1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser device with a current confinement oxide area, and more in particular to the surface emitting semiconductor laser device with the current confinement oxide area having a longer lifetime, or a higher reliability.
2. Description of the Related Art
A surface emitting semiconductor laser device emitting light in a direction perpendicular to a substrate attracts public attention in the data communication field because of the possible arrangement of a plurality of the laser devices in a two-dimensional array on a single substrate, different from a conventional Fabry-Perot semiconductor laser device.
The surface emitting semiconductor laser device includes a pair of DBRs (Distributed Bragg Reflector) (for example, Al(Ga)As/Ga(Al)As in the GaAs-based reflector) and an active layer acting as an emitting region sandwiched between the reflectors overlying a GaAs or InP substrate.
In order to increase the current injection efficiency and to reduce the threshold current, a surface emitting semiconductor laser device has been proposed having the current confinement structure formed by an Al oxide area.
For example, a GaInNAs-based material can be formed on the GaAs substrate, thereby providing a AlGaAs-based DBR mirror having an excellent thermal conductivity and a higher reflectivity. Accordingly, the mirror is promising in the surface emitting semiconductor laser device which emits light in a longer wavelength region from 1.2 to 1.6 μm.
As shown in FIG. 1, a conventional 850 nm-range surface emitting semiconductor laser device 10 includes a layer structure, overlying an n-GaAs substrate 12, having a bottom distributed Bragg reflector (hereinafter referred to as “DBR”) mirror 14 having 35 pairs of n-Al0.9GaAs/n-Al0.2GaAs layers each having a thickness of λ/4n (“λ” is a lasing wavelength and “n” is a refractive index), a bottom cladding layer 16, a quantum well active layer 18, a top cladding layer 20 and a top DBR mirror 22 having 25 pairs of p-Al0.9GaAs/p-Al0.2GaAs each having a thickness of λ/4n.
In the top DBR mirror 22, one of the layers close to the active layer 18 is formed as an AlAs layer 24 in place of the Al0.9GaAs layer, and Al of the AlAs layer 24 in the area other than a current injection area is selectively oxidized to form a current confinement area formed by an Al oxide area 25 which surrounds the current injection area.
The top DBR mirror 22 in the layer structure is configured to be a circular mesa post 23 having a diameter of 30 μm from the top to the layer near to the active layer 18 formed by the photolithographic and etching process.
The annular current confinement area made of the Al oxide area 25 is formed in the mesa post 23 by selectively oxidizing the Al in the AlAs layer 24 inwardly from the outer periphery of the mesa post 23 by means of the oxidation treatment of the layer structure at about 400° C. in a water vapor ambient. When, for example, the Al oxide area 25 includes an annular ring having a width of 10 μm, the surface area of the central AlAs area 24 or the surface area for the current injection (aperture) is about 80 μm2 having a circular shape with a diameter of 10 μm.
The mesa post 23 is surrounded by, for example, a polyimide section 26, and a ring-shaped electrode acting as a p-side electrode 28 is mounted in contact with the periphery of the top surface of the mesa post 23 by the width from 5 μm to 10 μm. After the thickness of the n-GaAs substrate 12 is adjusted to about 200 μm by polishing the bottom surface thereof, an n-side electrode 30 is formed thereon.
An electrode pad 32 for connection with an external terminal is mounted on the polyimide section 26 and in contact with the ring-shaped electrode 28.
In the conventional current confinement oxide layer structure, the conversion of the AlAs layer into the Al oxide area by the oxidation contracts the volume thereof to generate stress in the compound semiconductor layers adjacent to the Al oxide area. Thereby, the active layer is deteriorated to reduce the lifetime of the device because the active layer exists in the vicinity of the Al oxide area.
Thus, in order to prevent reduction of the device lifetime, use of an Al0.98Ga0.02As layer containing a smaller amount of gallium (Ga) has been proposed in place of the AlAs layer. Further, the prevention of the stress is attempted by decreasing the thickness of the AlAs layer to about 40 nm.
However, the oxidation rate is reduced by one order compared with that of the AlAs layer having an ordinary thickness of 60 nm when the Al0.98Ga0.02As layer or the thinner AlAs layer is used.
Accordingly, in order to obtain the Al oxide area having the same width in the Al0.98Ga0.02As layer or the thinner AlAs layer, the time of the oxidation should be increased or the oxidation temperature should be elevated.
As a result, as shown in FIG. 2, a problem arises that the Al0.9Ga0.1As layer having the higher Al content or the lower refractive index layer in the DBR mirror is oxidized in the annular shape along the periphery of the mesa post because the top DBR mirror is exposed to the severe oxidation conditions during the oxidation of the AlAs layer (current confinement layer).
The width of the oxide area formed in the DBR mirror depends on the oxidation conditions including the composition and the thickness of the Al oxide area for the current confinement and the composition and the thickness of the compound semiconductor layers in the DBR mirror, and at least about 2 to 5 μm is inevitably oxidized.
Although the oxidation amount of about 2 to 5 μm is smaller with respect to the diameter of the mesa post, the volume contraction is large enough to be neglected because the number of the Al0.9Ga0.1As layers constituting the DBR mirror is large and each of the layers has an increased film thickness even if the oxide width is only several μm. The stress generated due to the volume contraction may adversely influence the reliability of the surface emitting semiconductor laser device.
Since the thickness of the DBR mirror is λ/4n, the influence increases with the increase of the wavelength of the surface emitting semiconductor laser device.
Further, as shown in FIG. 3, when a force is applied to the mesa post in the direction of the arrow shown therein, the stress is generated on the boundary between the AlAs layer 24 and the Al oxide area 25 or the vicinity of the front edge of the Al oxide area 25 (near to the center of the mesa post, a region “C” shown in FIG. 3).
Actually, the specimen of the surface emitting semiconductor laser device which was compulsorily deteriorated through the reliability test was observed with a transmission electro-microscope to confirm the occurrence of rearrangement around the region “C” in FIG. 3.
The factors exerting the stress on the surface emitting semiconductor laser device includes the external factor and the internal factor. The external factors includes, for example, a dielectric, a protection film made of polyimide and an electrode. Further, in the mounting step of the surface emitting semiconductor laser device, the various forces may be applied to the mesa post.
The internal factor includes a distortion generated in the layer film after the crystal growth. Especially, the thickness of the layer structure having the top and bottom DBR mirrors amounts to 10 μm in the surface emitting semiconductor laser device, and the distortion in the layer structure including the compound semiconductor layers is not negligible.
Further, since the thickness of the compound semiconductor layers is established to be ¼n (wherein “n” is a refractive index) times the wavelength, the thickness of the DBR mirror is increased in the surface emitting semiconductor laser device having a longer wavelength range of 1.2 to 1.6 μm. The thickness of the layer structure increases with the increase of the DBR mirror to increase the distortion, thereby significantly exerting the adverse influence to the lifetime of the surface emitting semiconductor laser device.
A countermeasure for improving the device reliability includes the reduction of the stress applied to the front edge of the Al oxide area.